Well...
Plakat Betta sounds like a pretty nice Betta. I have also had mellow ones and more aggressive ones. Usually more aggressive only to other Bettas, though.
If he seems to want more room to swim around, sure, put him in the 15. If 77 degrees F is the right temp for the other fish the Betta ought to be fine. While they prefer "warm" water, this is usually said to people who are keeping them in unheated, room temperature tanks (about 70 degree water), so that "warmer" is a relative term.
You are right to question the optimum temperature for the other fish, though.
Endlers can be kept in ponds through the summer (here, SF bay area) but need to come in during the winter. So I suspect they have a much wider adaptability than they are given credit for. Cool room temperature or slightly warmer is just fine for them. A lot warmer might be a problem. I don't know about the hybrids, though. Probably not quite so cold.
You can research the various Rasboras and related fish at something like Fish Base. org to find out the original (wild) conditions they came from, but for the possible range the various aquarium sites are going to be better.
Most Cories tend to be somewhat cooler water fish, most being quite comfortable in the low to mid 70s, some down into the 60s, but few are tolerant of upper 70s. I would research Cories at Planet Catfish.
There is also the question of water parameters. I think all the fish you list are fine with moderately soft or softer water, except, perhaps the Guppy x Endler hybrids. I would do more research, but would tend to think slightly harder water might be better. Being a hybrid they might have inherited the softer water tolerance of Endlers or hatchery Guppies, or the hard water preference of wild Guppies.
Plakat Betta sounds like a pretty nice Betta. I have also had mellow ones and more aggressive ones. Usually more aggressive only to other Bettas, though.
If he seems to want more room to swim around, sure, put him in the 15. If 77 degrees F is the right temp for the other fish the Betta ought to be fine. While they prefer "warm" water, this is usually said to people who are keeping them in unheated, room temperature tanks (about 70 degree water), so that "warmer" is a relative term.
You are right to question the optimum temperature for the other fish, though.
Endlers can be kept in ponds through the summer (here, SF bay area) but need to come in during the winter. So I suspect they have a much wider adaptability than they are given credit for. Cool room temperature or slightly warmer is just fine for them. A lot warmer might be a problem. I don't know about the hybrids, though. Probably not quite so cold.
You can research the various Rasboras and related fish at something like Fish Base. org to find out the original (wild) conditions they came from, but for the possible range the various aquarium sites are going to be better.
Most Cories tend to be somewhat cooler water fish, most being quite comfortable in the low to mid 70s, some down into the 60s, but few are tolerant of upper 70s. I would research Cories at Planet Catfish.
There is also the question of water parameters. I think all the fish you list are fine with moderately soft or softer water, except, perhaps the Guppy x Endler hybrids. I would do more research, but would tend to think slightly harder water might be better. Being a hybrid they might have inherited the softer water tolerance of Endlers or hatchery Guppies, or the hard water preference of wild Guppies.
